In a mobile communication such as a cellular phone, in the case of applying a transmitting signal with non-constant envelope due to GMSK (Gaussian minimum shift keying) modulation, an offset PLL (phase locked loop) is frequently employed in the transmitter circuit. The offset PLL subjects a voltage control oscillator to modulation to convert a modulation signal into a transmission frequency. The detailed operation of the offset PLL is As disclosed in, for example, (Behzad Razavi), “RF Transmitter Architectures and Circuits)”, (U.S.), (IEEE 1999 Custom Integrated Circuits Conference) a draft book, 1999, pp. 197 to 204.
Also, in the case of applying a transmission signal of multi-levels and multi-phases (therefore, non-constant envelope) due to 8-value PSK (phase shift keying) modulation, etc., a double balanced mixer is frequently used in the transmitter circuit. The double balanced mixer is designed in such a manner that two pairs of transistors that turn on or off in response to a carrier signal are used, and the respective currents of those two paired transistors are changed according to a differential modulation signal, for example, as disclosed in JP 2001-223535A.